Hello. Thank you for writing to us at healthcaremagic
I will keep a possibility of Tinea/
Fungal infection.
Because of the anatomy, Tinea or fungal infections are common at
skin fold sites like under breasts; lower abdomen, waist (belt area), thighs are also commonly involved.
A 'KOH wet mount preparation' made from small amount of scale scraped from the surface, to look for fungal elements would be confirmatory.
You may try using an OTC
topical antifungal e.g
clotrimazole 1% cream twice daily for 2-4 weeks.
Since, it is extensive therefore I am in favor of Oral antifungals in addition to the topical antifungals.
Oral antifungals are prescription drugs, therefore, I suggest you to visit a
dermatologist in your region, if topical OTC antifungal cream alone is not very effective.
An an OTC oral antihistamine e.g cetrizine 10 mg once daily for symptomatic relief from discomfort.
Regards